This invention relates to composite microporous filtration membranes and to a process for producing the membranes. More particularly, this invention relates to composite microporous membranes made from a permeable nonretentive microporous substrate with a thin retentive microporous layer.
Microporous and open ultrafiltration membranes include thin sheets and hollow fibers generally formed from synthetic thermoplastic materials and having a substantially continuous matrix structure containing open pores or conduits of small size. The mean pore size range for pores of "microporous membranes" is not precisely defined in the art, but it is generally understood to extend from about 0.01 microns to about 10 microns. Microporous membranes having open pores thereby imparting permeability are useful in fine filtration. Ultrafiltration (UF) membranes have an average pore size less than that of microporous membranes and therefore are more retentive than microporous membranes.
Composite ultrafiltration (UF) membranes are UF membranes formed on a pre-existing microporous membrane substrate. The composite membranes have better integrity (higher bubble points) than UF membranes cast from the same polymer solutions onto traditional non-woven backing materials such as a non-woven polyester substrate. It is also disclosed in U.S. Pat. No. 4,770,777 to form a polyamide composite microporous membrane. A first layer of a polyamide solution is cast on a support and a fabric then is imbedded into the polyamide layer. A second layer of polyamide solution then is cast onto the fabric in the first polyamide layer. The first and second layers are then coagulated to form a microporous composite membrane where the pores in the first layer are larger than the pores in the second layer. The presence of the fabric layer is undesirable since it increases the risk of creating a non-retentive conduit in the composite membrane. It is also known to provide a composite membrane having a microporous layer and an ultrafiltration layer as disclosed, for example, in U.S. Pat. No. 4,824,568.
Most microfiltration membranes are manufactured by the so called immersion casting process. In this process, a polymer solution is cast into a film and immersed in a nonsolvent immersion bath. The polymer precipitates in this non-solvent and forms a porous structure. The pore sizes throughout the depth of this structure are determined by the formulation of the casting solution, properties of the non-solvent immersion bath and the parameters of the casting process. The filtration properties of these filters depend on the number of pores and the distribution of pore sizes. The process can lead to essentially isotropic structures where the distribution of pore size is approximately the same throughout the whole membrane depth from one surface of the membrane to the other surface. An example of isotropic microfiltration membranes is the Durapore product line to Millipore Corporation manufactured according to U.S. Pat. No. 4,203,847. In certain cases, the formulation of casting solution or the design of the casting process lead to anisotropic structures where distribution of pore sizes changes from one surface of the membrane to the other. An example of such membrane is the Filterite membrane marketed by Memtek USA, manufactured according to U.S. Pat. No. 4,629,563. The pore size in this membrane type change monotonically from one surface to the other. Another example of an anisotropic membrane is described in U.S. Pat. No. 4,933,081. The pore sizes in that membrane decrease from one membrane surface, reach a minimum inside the membrane structure and again increase toward the other surface of the membrane. Microfiltration membranes are used in general for separation of particles and colloidal matter from fluids. To accomplish this separation, they must contain within their structures at least one layer of pores capable of retaining the particulate matter and they must be mechanically strong to endure typical stresses associated with handling in device manufacturing and with the flow of fluids through their structure. A typical goal of membrane filter improvements is an increase of permeability at equivalent retention or vice versa. In general, small pores lead to a better retention but lower permeability. One way to improve performance is to decrease the thickness of the retentive part of membrane structure, containing typically the smallest pores. However, to maintain the mechanical strength, other layers of the membrane have to be present. These layers decrease the permeability of the membrane.
Accordingly, it would be desirable to provide composite microporous membranes which are retentive while maintaining high permeability of the composite membrane. In addition, it would be desirable to control independently the characteristics of a retentive layer and a nonretentive layer of a composite microporous membrane wherein the desired retentive characteristics can be maintained without adversely affecting the permeability of the nonretentive layer and while retaining structural integrity during use.